- According to the usage conditions, the design of a cryogenic valve has the following requirements:
- The valve should be able to work for a long time in low-temperature media and ambient temperature. The general service life is 10 years or 3000 to 5000 cycles.
- The valve should not become a significant heat source in the low-temperature system. This is because the inflow of heat not only reduces the thermal efficiency but also causes the internal fluid to evaporate rapidly if there is too much inflow, resulting in abnormal pressure increase and causing danger.
- Low-temperature media should not have harmful effects on handwheel operation and packing sealing performance.
- Valve assemblies that are in direct contact with low-temperature media should have explosion-proof and fire-proof structures.
- Valve assemblies operating at low temperatures cannot be lubricated, so structural measures need to be taken to prevent friction parts from being scratched.
- The factors that should be considered when selecting the main body material of the cryogenic valve include the following requirements:
The minimum operating temperature of the valve.
Metal materials maintain the mechanical properties required for working conditions at low temperatures, especially impact toughness, relative elongation, and structural stability.
The material has sufficient low-temperature impact strength.
Materials have different cold-shrink properties.
It has good wear resistance under low temperatures and oil-free lubrication conditions.
Has good corrosion resistance.
When welding is used, the welding performance of the material must also be considered.
The selection principles for materials such as the body, valve cover, valve seat, and valve disc of the main components of the cryogenic valve are: ferritic steel is used when the temperature is higher than -100°C; austenitic steel is used when the temperature is lower than -100°C, and low-pressure And small diameter valves can be made of materials such as steel and aluminum. Selection of materials for valve stems and fasteners: When the temperature is higher than -100°C, the valve stems and bolts are made of alloy steel such as Ni, Cr-Mo, etc., and undergo appropriate heat treatment to improve tensile strength and prevent thread bite; When the temperature is below -100°C, it is made of austenitic stainless acid-resistant steel. At the same time, the valve stem must be hard chromium plated (plating thickness 0.04mm~0.06mm) or nitrided to improve surface hardness. To prevent nuts and bolts from seizing, nuts are generally made of Mo steel or Ni steel, and molybdenum disulfide is coated on the thread surface. Gaskets for low-temperature valves must have reliable sealing and recovery properties at normal temperature, low temperature, and temperature changes. Since gasket materials will harden and reduce plasticity at low temperatures, gasket materials with small performance changes should be selected. When the operating temperature is -200°C and the low-temperature maximum operating pressure is 3MPa, the asbestos rubber sheet of long-fiber white asbestos is used. When the operating temperature is -200°C and the maximum operating pressure is 5MPa, a spiral wound gasket made of acid-resistant steel tape sandwiched with asbestos or a spiral wound gasket made of polytetrafluoroethylene acid-resistant steel tape is used. Wound gaskets made of expanded graphite and acid-resistant steel are ideal for low-temperature valves at -200°C. The selection of packing for cryogenic valves should consider the low-temperature characteristics of the packing. Generally, impregnated polytetrafluoroethylene V-type packing is used in cryogenic valves.
- The structural design of cryogenic valves has the following basic requirements:
- Use a valve body that can fully withstand expansion and contraction caused by temperature changes, and the structure of the valve seat will not be deformed due to temperature changes.
- Adopt a long-neck valve cover structure that can protect the stuffing box. The main purpose of adopting this structure is to reduce the heat in the transfer device and prevent the valve stem and upper parts of the valve cover from frosting and freezing due to overcooling of the stuffing box. Ensure that the temperature of the stuffing box is at 0 Above ℃
- Adopt a valve disc that can maintain reliable sealing regardless of temperature changes. For example, gate valves use elastic gates or open gates; stop valves use flat valve seats and needle valves use plug-shaped valve discs.
- Adopt upper sealing structure. Cryogenic valves are generally required to have a sealing structure. The upper sealing surface is generally overlayed with cobalt-chromium-tungsten carbide, and then ground after finishing.
- Use cobalt-chromium-tungsten carbide surfacing structure for the valve seat and valve disc (gate plate) sealing surface
- Note: The soft sealing structure has a large expansion coefficient and becomes brittle at low temperatures, so it is only suitable for cryogenic valves with temperatures above -70°C, but polytrichlorethylene can be used for cryogenic valves with temperatures up to 162°C. Cryogenic valves can also adopt a corrugated sealing structure without packing.
- Use pressure relief holes to prevent abnormal pressure rise. The location of the pressure relief hole depends on the valve structure, some are on the valve body, and some are on the gate plate. An outlet pipe can also be set on the valve or a safety valve can be installed to discharge abnormally high pressure.